1. Field of the Invention
The present invention generally relates to a resolution-converting apparatus and method. More particularly, the present invention relates to a resolution-converting apparatus and method preventing the occurrence of factors deteriorating image quality and converting an input image into an output image having a desired resolution size.
2. Description of the Related Art
The digital display devices, such as liquid crystal displays (LCDs) or the like, have a fixed screen resolution depending on products, unlike analog display devices, such as cathode ray tubes (CRTs), so resolution conversion is essential to convert the diverse resolutions of input images into a screen resolution of a display device. In particular, with the advent of high-definition televisions (HDTVs), the resolution conversion becomes more essential to display the images of existing standard definition-class resolution on a high-definition-class TV system.
However, the conventional methods used for such resolution conversions introduces image artifacts around image edges greatly affecting image perceptions through human eyes. There are the typical image artifacts such as the jagging artifacts of producing a sawtoothed pattern around image edges resulting from resolution conversions, the blurring artifacts of producing poor sharpness due to the resolution conversions, the ringing and aliasing artifacts around image edges due to poor filter characteristics, and so on. The conventional technologies related to the resolution conversions are mainly classified into a method using linear filtering technologies and a method using non-linear filtering technologies. That is, the method disclosed in U.S. Pat. Nos. 5,889,895 and 5,671,298 and U.S. Patent Application No. 2002/0140854 carry out resolution conversions using the linear filtering schemes, such as bilinear and cubic interpolations. However, such methods have a problem of deteriorating the image quality of output images since the sharpness becomes very poor due to the insufficient reproduction of the high-frequency components of an input image when upsizing resolutions. In order to compensate for such a problem, U.S. patent application No. 2002/0140854 discloses a method of outputting a high-definition image signal by identifying potential edge pixels with application of the peaking to a low resolution signal, applying the up-conversion with respect to the peaked image signal, and sequentially carrying out edge pixel detections, edge linking, and luminance transient improvements. However, the method enhances details and sharpness by using the existing linear filters during scaling and applying the peaking and luminance transient improvement functions based on the pre-processing and post-processing approaches before and after filtering, so the method needs lots of computations and a complicated hardware structure, therefore limiting the performance improvements.
The methods using the non-linear filtering techniques disclosed in U.S. Pat. Nos. 5,852,470 and 5,446,804 can be divided into the directional interpolation method and the content-based interpolation method. The directional interpolation method performs interpolation only in a direction parallel with an edge direction to prevent the edge components from producing blurring artifacts, using edge information of an image. The content-based interpolation method obtains an optimized scaling filter coefficient through a learning process in advance, selects a learned filter coefficient depending on local characteristics of an input image, and performs resolution conversions. Such methods provide relatively good results in the edge portions of an image, but has a disadvantage of degrading image quality in fine-textured regions.